While work is being carried out on the track, catenaries, etc. of double track lines, trains still go through in both directions on one track only. In general, the corresponding safety installations are lacking and when operating trains in both directions technical running safety has to be forfeited. In this article the possibilities that exist for overcoming these drawbacks by means of a modern safety technique are studied. A block system installation for double track lines is twice suggested as an all-purpose solution. The advantages offered by such a system are described.